Arsenic is an element that very often co-occurs in copper minerals and constitutes their specific impurity. In pyrometallurgical processes of roasting and smelting of copper concentrates, volatile arsenic compounds are released into the atmosphere, which, due to the toxicity of these compounds, constitutes a major threat to the environment. During the smelting process, most of the arsenic is removed as a volatile compound As4O6 at concentrations up to 0.5 mg/l, while only 0.04%-0.06% is removed in a solid, stable form with slags [Piret, 1999]. Apart from the volatile arsenic compounds, high concentrations of arsenic are also found in dusts. For this reason, it is very important from both an economic and an environmental point of view, to develop an effective method of controlled removal of arsenic form copper deposits and the products of their processing.
In order for the removal of arsenic from copper minerals to bring the expected economic and environmental benefits, this process must be conducted at the early stages of the copper deposits processing, such as flotation. The traditional copper flotation systems are insufficient and inadequate for separation and division of sulfide minerals containing arsenic (e.g. enargite Cu3AsS4 or tennantite (Cu,Fe)12As4S13) from copper sulfides not containing arsenic, present in the ores. An aid to the conventional flotation methods are the methods proposed in recent years. One of the methods relates to the selective oxidation of sulfides, based on the electrochemical properties of the separated compounds [Fornasiero et al., 2001]. Another method of selective flotation utilizes the differences in the flotation pulp potentials [Guo and Yen, 2005]. Using the separation method based on the difference in the pulp's potentials, minerals containing arsenic, e.g. enargite (Cu3AsS4), can be separated from copper sulfides not containing arsenic. As a result of these processes, two fractions of concentrates are produced: (i) with a low arsenic content, and (ii) with a high arsenic content. The former can be used in pyrometallurgical processes, whereas the latter fraction of concentrates, containing copper minerals contaminated with arsenic, still requires adequate treatment [Senior et al., 2006].
One of the ways that can help to solve the problem of removal of arsenic from copper minerals, is the application of biohydrometallurgical methods, using microorganisms to recover metals from minerals and deposits. The use of microorganisms for the extraction of copper or gold from their ores and concentrates, is a well-known process, and is often described in the literature [Xia, L. et al., 2010; Xia, L. et al., 2009; Olson et al., 2003; Rawlings and Johnson, 2007]. Most of the biohydrometallurgical processes are based on the processes of oxidation of minerals, and lead to (i) an increase in the accessibility to chemical solvents (biooxidation) or (ii) their direct dissolution (bioleaching) [Rawlings and Johnson, 2007]. Unfortunately, these methods are non-specific, because they are based on the oxidation of sulfur and/or iron from minerals, and are associated with the release of all the metals associated with this type of minerals. A further limitation of the traditional biohydrometallurgical methods is the use of acidophilic bacteria in leaching from neutral or slightly alkaline deposits, which is often inefficient, and sometimes even impossible, due to the need for the use of considerable amounts of sulfuric acid to acidify the deposits. In the literature microorganisms are described, mainly chemolithoautotrophic, sulfur oxidizing bacteria, belonging to the genus: Thiobacillus, Halothiobacillus, Thiomonas, and iron oxidizing bacteria, such as: Galionella feruginea or Leptothrix ochracea, Thiothrix and Beggiatoa, which can be used in bioleaching processes at neutral pH, but are very difficult to cultivate and are still poorly understood. Furthermore, bioleaching with the use of these microorganisms is time-consuming and these processes are carried out as long as several months [Sklodowska and Madakowska, 2007]. A confirmation of the lack of suitable microorganisms, capable of recovering metals under neutral or slightly alkaline conditions is the current situation in the mining market. Currently, there are no known and commercially available bitechnological methods of removing precious metals, occurring in the form of sulphides, under the conditions of neutral or slightly alkaline deposits.
An alternative to the oxidation processes are selective bioreduction processes, in which the selected elements, associated with the metabolic activity of microorganisms, are released. Although there are several examples of application of the bioreduction processes, methods for removing arsenic from copper minerals using microbial reduction are unknown. Many strains of bacteria that dissimilatory reduce arsenates have been identified, but the use of most of them is limited to the transformation of soluble arsenic compounds [Newman et al., 1998] or secondary arsenic minerals, resulting from iron compounds [Zobrist et al, 2000]. Strains capable of removing arsenic from copper concentrates and flotation tailings have also been described [Mantur et al., 2011], but the dissimilatory arsenate reduction process, carried out by the aforementioned strains, is not fully balanced and part of the arsenic may be removed out of the cells in the form of volatile, toxic arsenic compounds (unpublished data). Apart from that, the strains described by Mantur et al., 2011, simultaneously release copper and arsenic from minerals, thus lowering the value of the obtained copper concentrate.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.